Now, let me re-arrange some collection of thoughts and theories regarding Detective Conan’s time flow and such.
When series first start off, Kudo Shinichi was 16 years old, a high school student of Teitan High.
Check point 1: Sometime in (late) April but before May 4th
*Japanese school begin first term in April, exact date varies upon the school (Anywhere between Early April to Late April)
**Ran birthday’s have yet to passed, she is still 16 (refer to stuff below)
Later on throughout the series (beginning of episode self intro, etc.) he referred to himself as 17 years old. The only time when his birthday has actually passed
(if I remember correctly) is in the 1st Movie “The Time Bombed Skyscraper". Assuming that the movie actually follows the storyline and is part of the time flow, Shinichi has turned 17.
Check point 2: The story has passed May 4th
Fast forwarding to "The Shadow of the Black Organization" arc occurred at during New Year
(and up to Setsuban around late January, beginning of February), where later on Hondo Eisuke disappeared which is however, said to be around 18th/19th of December (somehow this happen before the case… wtf?); At this point, Ran, Sonoko, Conan, and crew are still in the same grade meaning this occurred the same year Shinichi turned into Conan.
Check point 3: December 18th/19th to around New Year
*Somewhere between mid-December to mid-January
**So far, 8-9 months have passed canonically according to the time flow of Detective Conan.
Sometime afterward, the Detective Boys met with Jodie at a Cherry Blossom festival (Plot relevant). Sakura usually blooms in Japan around Late March to April. With the exception of Naha where Sakuras, on average, blooms (open to full) around January to early February.
Check point 4: Around mid-January to early-February of next year (the year after Shinichi turned into Conan)
*Still same academic year
**Ran is still 16
***Around 9 to 10 months have passed since the famous incident (a.k.a. beginning of Conan)
Japanese school academic year end in March. So far it is around mid-Jan to early-Feb, about a month of so left till end of school year.
Ran’s birthday has yet to come. She once mentioned in the manga that her birthday a ways a way from her mother (October 10th). In the first movie, when looking at the horoscope for Taurus, (April 20 to May 20) she mentioned how she and Shinichi share May’s lucky color, red. This means that Ran’s birthday is somewhere from mid-Feb
(or as early as mid-Jan) up to late-April.
Whatever her birthday actually is, it is highly possible that Ran’s 17th birthday will play a major role to the plot of the series.
To summarized the whole thing, it would seems that so far, around 9-10 months have passed since the beginning of Detective Conan where Shinichi turned into Conan.
**Updated as of Detective Conan chapter 868**